In the semiconductor device manufacturing process, a reduction projection exposure apparatus, which reduces the pattern formed on a mask and transfers the reduced pattern onto a substrate, is adopted. In order to achieve further micronization of the pattern to be transferred, techniques for increasing the resolving power of the exposure apparatus are proposed. In such techniques, a method of shortening the wavelength of exposure light and a method of increasing the numerical aperture (NA) of a projection optical system are generally used. As the numerical aperture of the projection optical system increases, its depth of focus decreases. The relationship between these factors can be generally expressed by:(resolving power)=k1λ/NA (depth of focus)=±k2λ/NA2 where λ is the wavelength of exposure light, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients.
As for the wavelength of exposure light, an ArF excimer laser, which emits light having an oscillation wavelength of around 193 nm, and even a fluorine (F2) excimer laser, which emits light having an oscillation wavelength of around 157 nm, have been developed. Independently of these lasers, a liquid immersion method has received a great deal of attention as a technique for increasing the resolving power. Conventionally, the space between the substrate and the lowermost surface of the projection optical system is filled with gas. However, the immersion method executes exposure while the space is filled with liquid.
Letting λ be the wavelength of exposure light in the air, and n be the refractive index of liquid used for liquid immersion, the above-described resolving power and the depth of focus are expressed by:(resolving power)=k1(λ/n)/NA (depth of focus)=±k2(λ/n)/NA2.
That is, the effect of liquid immersion amounts to using exposure light having a wavelength of 1/n. In other words, when a projection optical system uses the immersion method, its depth of focus increases to n times if its NA remains the same. This applies to any pattern shape. Moreover, this method can be combined with a currently proposed phase shift method or a modification illumination method.
In recent years, the immersion method has been attempted to be applied to exposure apparatuses. Japanese Patent Laid-Open No. 6-124873 discloses an example of a conventional exposure apparatus to which the immersion method is applied. This exposure apparatus fills a liquid tank with liquid, thereby filling the space between the wafer and the lowermost surface of the projection optical system. PCT International Publication No. WO 99/049504 discloses another example of the conventional exposure apparatus. This exposure apparatus uses a method of filling, with liquid, only the space between the projection optical system and the wafer surface.
An exposure apparatus unloads one wafer from the wafer stage after the wafer is exposed with a predetermined shot layout, and loads a new wafer to repeat the exposure operation. In a liquid supply method of a liquid immersion exposure apparatus, which executes exposure while the space between the projection optical system and the substrate is filled with liquid, liquid supply is stopped in wafer exchange. A new wafer is then loaded under the projection optical system. When the apparatus is ready for exposure, it restarts the liquid supply. In the scan type exposure apparatus, a direction in which liquid is supplied may be changed in accordance with the scanning direction. When such a scan type exposure apparatus switches nozzles for liquid supply in accordance with the scanning direction, the individual nozzles intermittently supply the liquid. Moreover, liquid supply must be stopped for every step of driving of the wafer, whether the apparatus is the scan type or not. In any other operation mode, such as a measurement mode for alignment or focusing, liquid supply must be temporarily stopped.
Assume that a pump for liquid supply is controlled to control the start/stop of the liquid supply. In this case, during intermittent stop of the liquid supply, gas or dust may mix in the liquid through a portion in a liquid supply nozzle, where the liquid surface is exposed to the air. Exposure using the liquid with gas or dust worsens the liquid quality uniformity, generates air bubbles, varies the photosensitive characteristic of the resist, or shields the optical path. These factors degrade the imaging performance and contaminate the optical components, such as the lens surface, which come into contact with the liquid, resulting in a decrease in light amount. That is, in the semiconductor device manufacturing process using the immersion exposure apparatus, it is necessary to prevent gas, such as air, from dissolving in, or dust from mixing in, liquid to be supplied between the projection optical system and the substrate.
Moreover, repetition of the start/stop of the liquid supply largely varies the flow rate of the liquid which flows in the liquid supply device, and flows through the path from the liquid supply device to the supply nozzle. This largely varies the flow rate of the liquid, which passes through temperature control mechanisms, such as a heater and a temperature sensor, for liquid temperature control. This makes it difficult to control the temperature of the liquid supplied between the projection optical system and the substrate. Unsatisfactory temperature control adversely affects the imaging performance due to a variation in refractive index of the liquid supplied.
Assume that a pump of the liquid supply device controls the start/stop of the liquid supply. This structure inhibits the productivity from increasing, because a response delay is large, and flow rate stabilization takes a long time.